It is known to oxidatively dehydrogenate ethane resulting in ethylene, in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. Examples of ethane ODH processes are for example disclosed in U.S. Pat. No. 7,091,377, WO2003064035, US20040147393, WO2010096909 and US20100256432. The oxidative dehydrogenation of ethane converts ethane into ethylene. In this process, ethane is reacted with oxygen in the presence of an ODH catalyst to produce a product stream comprising predominately ethylene, along with unreacted reactants (such as ethane and oxygen), and typically other gases and/or by-products (such as carbon monoxide, carbon dioxide, water).
In general, the yield of ethylene in an ODH process is reduced by the undesirable combustion reactions of ethane and ethylene, both of which are highly exothermic and generate carbon dioxide and/or carbon monoxide. As is generally the case in such exothermic processes, it is important to control the reaction temperature within a certain range to maintain effective and safe plant operation and also to extend the life of the catalyst and inhibit undesirable side reactions. It is known that a multitubular fixed-bed reactor may be used to conduct such exothermic reactions, with the reactor employing a plurality of tubes containing a fixed bed of catalyst particulates, and a shell in which the tubes are contained through which coolant circulates to facilitate the removal of the reaction heat.
Typically, it is desirable to maintain isothermal conditions on the coolant side of the reactor. This is usually accomplished either by using a boiling medium (e.g. water/steam, kerosene) as the coolant, wherein the low-temperature incoming feed gas is preheated to the reaction temperature at the expense of the coolant which enters the shell at a higher temperature, or by circulating a coolant that is in counter-current flow with the flow of the reactants through the tubes at a sufficiently high circulation rate so as to rapidly remove heat. However, fixed bed reactors used in exothermic reactions may nevertheless have the propensity to develop one or more “hot spots” in various regions of the reactor.
In an attempt to avoid the undesirable formation of a so-called “hot-spot” (a localized temperature peak) in the catalyst bed, one commonly proposed solution is to reduce the diameter of the tubes in order to increase the heat transfer rate per unit volume of the catalyst. However, this typically increases the cost associated with building the reactor and also increases the amount of time required to load and unload the catalyst into the tubes. Similarly, it may also limit somewhat the size/shape of catalyst that can be used. Likewise, if the lengths of the tubes are significantly increased, the pressure drop across the reactor may also undesirably increase. Another commonly proposed solution is to operate at a lower productivity or lower conversion, for example by diluting the catalyst with an inert substance. However, this also has the disadvantage of increased cost and typically increases the difficulty of later recovering the spent catalyst from the reactor for regeneration, if desired.
Accordingly, the present inventors have sought to provide improved processes for the oxidative dehydrogenation of ethane. In particular, the present inventors have sought to provide ODH processes that utilize a multitubular fixed-bed reactor wherein the generation of hot-spots in the catalyst bed is avoided or reduced, thereby preventing or minimizing the risk of a reactor runaway.